chroot

This function works like the system call by the same name: it makes the
named directory the new root directory for all further pathnames that
begin with a / by your process and all its children. (It doesn't
change your current working directory, which is unaffected.) For security
reasons, this call is restricted to the superuser. If FILENAME is
omitted, does a chroot to $_
.

NOTE: It is good security practice to do chdir("/") (to the root
directory) immediately after a chroot().